| Summary: | Density Functional Theory (DFT) method was adopted to investigate and compare the reaction mechanisms of ethylene polymerization catalyzed by neutral, cationic bis(imino)pyridyl (PDI) iron and cobalt derivatives. The electronic structure and the oxidation states of the metal center and the PDI ligand were analyzed by taking spin states, natural bond orbital (NBO) charge distribution, etc. into consideration, revealing that the reactivity is closely related to the valence electron numbers instead of the charge numbers. The neutral Co(0) had the lowest reactivity as it possessed the most electrons. During the formation of the cationic Co(+)/Fe(+), one electron was mainly lost from PDI ligand rather than the metal center while the metal center maintained +II valence state through the process. Moreover, a special unsymmetrically bidentate <i>N^N</i> coordination manner was found to provide the deficient metal surroundings with 14e, which may initiate the reactivity of some unsymmetrical species with rich electrons. Finally, an anion [AlMe<sub>4</sub>]<sup>-</sup> participating process was proposed to explain the presence of the experimentally observed LCo(+)B(C<sub>2</sub>H<sub>4</sub>). A special intermediate, Co(+)B(C<sub>2</sub>H<sub>4</sub>) [AlMe<sub>4</sub>]<sup>-</sup> with Co in +I and absence of Co–C σ bond, was obtained. These calculation results may provide fundamental information for further understanding and designing the ethylene polymerization catalysts.
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